Goods and other items can be tracked and identified using a radio frequency identification (RFID) system. The RFID system includes an RFID tag, which is placed on the item to be tracked. The RFID tag is a small transponder that can be read by an RFID interrogator. The interrogator includes a transceiver and an antenna. The antenna emits electromagnetic (EM) waves generated by the transceiver, which, when received by the tag, activate the tag. Once the tag has been activated, the tag can modify and reflect the waves back to the interrogator, thereby identifying the item to which the tag is attached or is otherwise associated with.
The interrogator may be a hand held or stationary device that transmits a radio signal which may be intercepted by the tag. When the tag passes through the radio waves, the tag detects the signal and is activated. Data encoded in the tag can then be transmitted to the interrogator for further processing. This system allows for quick and easy identification for a large number of items by simply passing them through the scope of an interrogator. This system will also identify items on which tag is not exposed, such as items in which the tag is located internally. Further, the interrogator can read multiple tags very quickly, such as items passing by the interrogator while the items are on a conveyer belt.
There are three basic types of RFID tags. A beam-powered tag is a passive device which receives energy required for operation from the radio waves generated by the interrogator. The beam powered tag rectifies an EM field and creates a change in reflectivity of the field which is reflected to and read by the interrogator. A battery-powered tag still receives and reflects EM waves from the interrogator, however the battery powered tag includes a battery to power the tag. An active tag actively transmits EM waves which are then received by the interrogator.
A typical interrogator may have a range of less than 10 meters, but the range can extend to more than 200 meters. The strength of the signal transmitted by the interrogator is one factor determining its range. Another is the tag's alignment with the axis of polarization of the transmitted signal. One way to improve the range of the interrogator is to have the antenna on the tag aligned with the axis of polarization of the antenna on the interrogator. For example, a conveyer belt may send a number of similar or identical boxes past an interrogator, and the RFID tags in the boxes can be aligned with the axis of polarization of the antenna on the interrogator. As a further example, the antenna on the tag may be placed horizontally along the inside of the box, and the interrogator may transmit along that horizontal plane.
In the above-mentioned example, it is previously known what the orientation of the items, and therefore the tags, will be. A common type of interrogator includes an antenna that transmits a signal that is predominantly linearly polarized, with this polarization oriented in a single direction. A tag will have an optimum orientation of its antenna to this polarized signal. If a tag happens to be positioned with its antenna at a ninety-degree angle relative to this optimum orientation, the communication range might only be one-twentieth the range of a properly aligned tag. As a result, when the orientation of items passing the interrogator is not known, the tags that are not aligned with the polarization of the interrogator may not be read. Some interrogators include an antenna that transmits a circularly polarized signal. However, to generate this circularly polarized signal, the strength of the outputted EM wave is significantly reduced. Other interrogators include two antennas; one to transmit horizontally polarized signals, and another to transmit vertically polarized signals. However adding the second antenna not only increases the complexity and cost of the interrogator, but also the size. What is needed is a simple and compact interrogator for reading RFID tags of differing orientations.